A photovoltaic cell, i.e., a solar cell means a device by which solar energy can be converted into electric energy. Particularly, when light is input to the photosensitive material contained in the photovoltaic cell, electrons and holes are generated by the so-called photovoltaic effect, thereby producing electric current and voltage. Since such a photovoltaic cell can obtain electric energy from pollution-free solar energy that is source of all types of energies, active studies have been conducted about photovoltaic cells in view of development of alternative energy.
Solar cells are classified into various types depending on materials used for a light-absorbing layer. Currently, silicon solar cells using silicon are used most frequently. However, while cost of silicon has been increased rapidly due to the lack of silicon supply, many attentions have been given to thin film solar cells. Since thin film solar cells are manufactured to have a small thickness, they require low consumption of materials and have a low weight, and thus can be applied to various industrial fields. As a material for a thin film solar cell, amorphous silicon, CdTe, CIS, CIGS or CIGSSe have been studied intensively.
A CIS, CIGS or CIGSSe thin film is one of the I—III-VI compound semiconductors including elements such as Cu, In, Ga, S, Se, or the like, and provides the highest conversion efficiency among the pilot thin film solar cells. Particularly, such a thin film solar cell can be manufactured to have a thickness of 10 micrometers or less, is stable even during long-term use, and thus is expected to be an economic high-efficiency solar cell capable of substituting for a silicon solar cell.
Particularly, CIGSSe having a high absorption coefficient and structural stability may be referred to as the most spotlighted light-absorbing layer as a material succeeding to a silicon solar cell. In addition, CIGSSe has an advantage in that it can realize absorbing layers having different band-gaps through the control of relative ratio of In to Ga and S to Se. A CIGSSe solar cell is driven by the junction of a p-type semiconductor and n-type semiconductor, and the operation of a CIGSSe solar cell largely depends on the band-gap and doping degree of each material. Recently, many studies have been conducted to improve transfer and operation of electrons by applying element concentration distribution or band-gap grading technology to a heterojunction structure as mentioned hereinafter.
In the case of a conventional solution process-based CIGSSe solar cell, it is expected that introduction of such a grading structure overcomes the problem of efficiency relatively lower than the efficiency of a vacuum process-based solar cell. In the case of an element deposition process based on a vacuum process, it is possible to form a concentration grading of the elements in a thin film with ease by controlling the proportion of a deposited element during the deposition process. However, when preparing a CIGSSe thin film based on a solution process, there is a problem in that it is relatively difficult to control the concentration distribution of elements in a thin film and to adjust a band-gap.